Add GitLabCredentials placeholder text for token field

[serinamarie]

Part of closing PrefectHQ/prefect#10468

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• References any related issue by including "Closes #" or "Closes ".
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• Includes tests or only affects documentation.
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[desertaxle]

I believe that all our consumers of this block append oauth2: to the token when necessary. If that's true, is this change necessary?

[serinamarie]

As requested in the issue, if not placeholder text we could include a better description since this is a recurrent issue.

Would something like this make more sense?

[serinamarie]

@desertaxle
Community issues:

GitLab
https://prefect-community.slack.com/archives/CL09KU1K7/p1689115696739679?thread_ts=1689088572.689859&cid=CL09KU1K7

https://prefect-community.slack.com/archives/CL09KU1K7/p1690867549084869?thread_ts=1689088572.689859&cid=CL09KU1K7

Some users are just using GitHubCredentials blocks as a workaround instead, which isn't ideal:
https://prefect-community.slack.com/archives/CM28LL405/p1692025643321819?thread_ts=1690989437.858689&cid=CM28LL405

There are more such issues for BitBucket as well.

But users do ask in multiple places in community for documentation on it.

[desertaxle]

In those threads, I see that our git_clone step wasn't formatting the URL, and that burden was placed on the user. You've improved the git_clone step to handle this for users seamlessly, so whether users prefix their tokens has no impact on functionality. That makes me think this change isn't needed, but let me know if there's a use case I'm not considering.

[serinamarie]

I was thinking if we were to add other deployment steps!

However, with the new _format_token_from_access_token and _format_token_from_credentials helper functions, it shouldn't be hard to do the same thing with similar deployment steps using git. I'll close this then.